It is known to prepare amorphous or crystalline polyesters having properties adapted to a particular application, from polyfunctional alcohols, i.e., diols, triols or polyols and polybasic carboxylic acids, i.e., di-, tri- or polycarboxylic acids or their derivatives.
If a multi-step synthesis is performed, the first step can be a transesterification reaction using low alkyl esters, especially methyl esters, of the carboxylic acids, the second step can be an esterification reaction involving the use of carboxylic acids, and the third step can be a polycondensation reaction. If esters of carboxylic acids are not used, the preparation begins with the esterification. Each of these steps requires, as a rule, different catalysts, since the zinc, cadmium, manganese or calcium transesterification or esterification catalysts either do not accelerate the esterification or polycondensation reaction sufficiently, or, in the higher concentrations necessary for the esterification or polycondensation reaction, which are higher than those required for the transesterification or esterification reaction, they produce not only the polycondensation reaction but also degradation reactions resulting in discoloration, among other undesirable effects. But as the percentage of degradation products in the polycondensates increases, their useful properties are impaired.
Consequently, in the state of the art, the concentration of the transesterification and/or esterification catalysts is kept at a low level that is only high enough to provide for the smooth course of the reaction, or the catalyst of the transesterification step is deactivated after the reaction, for example by the addition of phosphorus compounds such as phosphoric acid, phosphorous acid, hypophosphorous acid, phosphinic acid, or esters or salts thereof, and the polycondensation is performed after a different catalyst has been added.
Large numbers of catalysts suitable for the polycondensation are specified in R. E. Wilfang, J. Polymer Sci. 54 (1961) 388. Soluble antimony compounds are frequently used, especially for the polycondensation on account of their good catalytic action. They have the disadvantage, however, that they easily react to form metallic antimony, which gives a gray discoloration to the polycondensate.
Compounds of other metals, however, are less active polycondensation catalysts, they require greater molar amounts or longer reaction times before the desired molecular weights are reached, they catalyze secondary reactions or degradation reactions more than antimony compounds do, or they necessitate the addition of the above-named phosphorus compounds for deactivation.
It is also known to use soluble titanium compounds as polycondensation catalysts. According to H. Zimmermann: Faserforschung u. Textiltechnik 13 No. 11 (1962), 481 to 490, their catalytic activity is comparable to that of antimony compounds, and the secondary reactions are tolerable. Titanium compounds, however, have the disadvantage of giving the polycondensates a yellowish-brown discoloration, especially when ethylene glycol is a component of the polyesters.
The problem therefore existed of reducing the polycondensate discoloration which is produced by titanium catalysts and of increasing insofar as possible the activity of titanium catalysts.